The invention relates to an optical spectrometer system which detects the intensity of light as a function of wavelength.
Optical spectrometers are commonly used for the detection and subsequent analysis of light signals. The optical spectrum is what the human eye sees when light is diffracted by water in the atmosphere and forms a rainbow. Each color of light in the rainbow is defined by its own wavelength. The human eye actually measures the wavelength and intensity of light when it "sees" color and brightness. However, in order to measure the composition of light consistently and precisely, an optical spectrometer is generally used. The optical spectrometer measures intensity, or brightness of light as it relates to wavelength, or colors contained within the light.
This optical spectrum analysis is commonly used for chemical analysis, optics research and optics sensor development. Other applications include paint matching, quality control for packaging, food, cosmetics, color cathode ray tubes, automobile tail light lenses, etc.
Typically, an arrangement of entrance slits, lenses, mirrors and gratings are used to align, project, chromatically disperse and focus light signals to be analyzed into a spectral band which is projected onto a photodetector array. The spectral range and resolution of the spectrometer are governed by the focusing power of the lenses or curved mirrors, the resolution of the grating, the entrance slit width, the number of photodetectors within the photodetector array and the individual photodetector element spacing.
In the special case of a monochronomator, the light signals to be analyzed are passed through a narrow entrance slit or hole, projected onto a diffraction grating via a lens or curved reflective mirror, and refocused via a lens or curved reflective mirror into a spectral band which is projected through a narrow exit slit or hole onto a single photodetector. The grating is rotated, projecting a narrow wavelength band of light through the exit slit or hole, the center wavelength of which is a function of the grating angle.
Disadvantages of the spectrometers and monochronomators described include large physical size and susceptibility to contamination.
An optical micro-spectrometer is described in U.S. Pat. No. 4,198,117. This device utilizes a graded index (GRIN) lens along with a dispersion grating and optical fibers. This device offers a great reduction in size and is not susceptible to contamination since the path of the signal light always remains within a solid optical path. Yet this device has the disadvantage of a limited resolution/total spectral bandwidth product. As one attempts to achieve higher resolution, the total spectral bandwidth is reduced, and vice versa. Limiting constraints in this device also include maximum aperture of the lens which can be fabricated, focal length variation as a function of wavelength, and large overall physical size required for a device designed to cover a wide optical bandwidth while having fine spectral resolution.